The block size for Bitcoin has been a problem for a while. It's not big enough (1MB) to fit in all the transactions that users want to make.
Therefore there is a huge backlog of transactions waiting for a block increasing the transaction times. Making the whole system slow.
(You can compare it to a train station where a train arrives every 10 minutes and has not enough wagons for all the passengers to get in. But new passengers are arriving all the time. Before you know it you have a line outside the station.)